Tunable nonreciprocal coupling network



July'7, 1970 MASAHIRO oMoRx 3,519,957

TUNABLE NONRECIPROCAL. coUPLING NETWORK Filed Sept. 27, 1968 2 Sheets-Sheet 1 SOURCE Byh A T TORNE y July 7, 1970 MASAHIRQ oMoRl 3,519,957

TUNABLE NONRECIPROCAL couPLING NETWORK Filed Sept. 27, 1968 2 Sheets-Sheet 2 l FREQUENCY 4GUTPUT NULL 3,519,957 TUNABLE NONRECIPROCAL COUPLING NETWORK Masahiro Omori, Palo Alto, Calif., assignor to Bell Telephone Laboratories, Incorporated, Murray Hill, NJ., a corporation of New York Filed Sept. 27, 1968, Ser. No. 763,302 Int. Cl. H0111 1/32; H03h 7/44 U.S. Cl. S33-1.1 7 Claims ABSTRACT OF THE DISCLOSURE A multiport nonreciprocal coupling network in which the ports are each coupled to two resonators which support rotating electromagnetic fields at different phase angles when excited at an operating frequency intermediate to the resonant frequencies. The coupling is arranged so that the superposition of the two fields results in a null at one port and isolates that port. Independent adjustment of the resonant frequencies allows selection of the operating frequency.

BACKGROUND OF THE INVENTION This invention relates to nonreciprocal coupling networks and more particularly to tunable circulators.

Circulators are now well known and practically indispensable components in the microwave transmission art. In general, they are nonreciprocal junctions having coupling characteristics which derive their nonreciprocity from a magnetically polarized element of gyromagnetic material located in the junction. Usually the gyromagnetic element is ferrimagnetic material such as ferrite or garnet. One of the popular circulator forms is the split mode, three branch or Y junction type, the basic principles and uses of which may be found in various texts such as Microwave Ferrites and Ferrimagnetics by Lax and Button, 1962, pages 517 and 609; in publications such as Fay and Comstock, Operation of the Ferrite Junction Circulator, 13 IEEE, Transactions MT & T, pages 15-27, January 1965; and in U.S. patents such as 3,063,024 issued to L. Davis, Jr., Nov. 6, 1962.

As described in these references the operating frequency of the split mode circulator depends upon the D.C. magnetic biasing field and the geometry of the ferrimagnetic material. Split mode circulators are normally broadband and only very limited frequency adjustment on either side of a center frequency can be attained by varying the intensity of the single D.C. magnetic field. Significant variation beyond the predetermined band of operation normally requires changing the geometric properties of the device. The operating range is limited by the upper and lower resonant frequencies of the split modes and these frequencies cannot be independently controlled by control of the magnetic biasing field because they are mutually affected by variations in the biasing of the single resonator in which the two modes exist. For certain applications, such as channel dropping, a narrow band variable frequency circulator is desirable. Using conventional broadband split mode circulators, a serially connected tunable filter such as a narrow band cavity would be required for such an application. A more economical solution would be, of course, to replace this tunable filtercirculator combination with a single tunable device.

SUMMARY OF THE INVENTION It is an object of this invention to provide a narrow band circulator and it is also an object of this invention to provide a circulator which is easily tuned over a wide frequency range without alteration of the physical properties of the device.

United States Patent O ICC Two resonators are provided, each supporting one of the two oppositely rotating eletcroma'gnetic fields or modes which are required for circulator operation. Because the two resonators are physically distinct, it is possible to achieve independent adjustment of the resonant frequency of each. By adjusting the two resonant frequencies, while maintaining a preselected phase difference between the modes, the center frequency of the operating range, which is intermediate to the resonant frequencies, may be altered. If this dual resonator circulator is designed for narrow band operation the tuning of the center frequency will alter the operating range from one frequency band to another.

The resonance may be adjusted in any suitable manner. If, for example, the resonators are ferrimagnetic resonators, such as spheres of ferrite, a variation of intensities of their respective D.C. magnetic biasing fields will effect a variation in the resonant frequencies. If the resonators are of some other type and have nonreciprocal characteristics derived other than from ferrimagnetics, their resonance will be suitably adjusted by use of D C. magnetic fields, tuning stubs or other known means.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a representation, partially schematic and partially pictorial, of a circulator in accordance with the present invention in which each of the dual resonators is an independently magnetically biased sphere of ferrimagnetic material;

FIG. 2 is a graphic representation of the transmissionfrequency characteristics of the present invention, and the amplitude-frequency and phase-frequency characteristics of the dual resonators; and

FIG. 3 is a schematic representation of the circulator of FIG. 1 showing the coupling orientation of the ports.

DETAILED DESCRIPTION In the presence of a D.C. magnetic biasing field a ferrimagnetic material exhibits precession of magnetic moments at a natural or resonant frequency which is a function of the material and the intensity of the bias field. This precession tends to be damped out by internal forces. Electromagnetic energy, which will be referred to as RF energy, transmitted such that its magnetic field rotates (is circularly polarized) in the same direction as the precession will be absorbed by the material. This absorbed energy will impede the damping action so that precession is maintained. A magnetic field rotating in a direction opposite to the precession will pass through the material without any appreciable effect upon the precession.

ln accordance with this invention, two ferrimagnetic resonators are provided into which the same RF energy is introduced. Each resonator is magnetically biased in a different direction relative to the introduced energy and each resonator supports a single uniform precession mode of the RF wave. Each mode rotates in an opposite direction because their resonators are oppositely biased.

The intensity of a D C. magnetic field does, as is well known, control the precession frequency, and the independent magnetic biasing of each resonator permits independent control of the frequency of each resonator. The appropriate frequency of the RF energy which will be supported in such resonators is limited by their resonant frequencies. Thus, adjusting these resonant frequencies tunes the operating RF frequency.

As is well known by those skilled in the art of circulators, the impedance of the rotating mode associated with the higher resonant frequency will have an inductive reactance component and that associated with the lower resonant frequency will have a capacitive reactive component at an intermediate frequency of the two resonant frequencies. In a three port device if the phase angles of the impedances of the t-wo modes are separated by 60, as for instance a 30 lead in one resonator and a 30 lag in the other, for an input at the operating frequency, the voltage pattern produced by superposition of both modes will produce a null at one of the ports, thus causing circulation from the input port to the other non-null port. The direction of circulation is the direction of the precessional resonance of the resonator having the lower D.C. magnetic biasing field.

Referring to FIG. 1 resonators 10 and 11, such as spheres or ellipsoids of ferrimagnetic material, as for example ferrite or garnet, are each individually biased by D C. magnetic fields Hd A and Hdc B which are illustrated as emanating from electromagnets 12 and 13, respectively. These magnetic fields may, of course, be provided by alternative means such as a permanent magnet and the magnetic intensities can, of course, be adjusted by such means as varying the effective size or separation of the pole faces and 22 or 21 and 23. The intensities of Hdc A and Hdc B may also be varied as shown by adjusting common potentiometer 14 which controls the current from D.C. source 16. This adjustment of current through coils 18 and 19 causes variation in the intensities of both fields. In addition, Hdc B can be varied separately by adjustment of potentiometer 15 which controls the current through coil 19. Initial adjustment of Hdc B by potentiometer 15 is necessary to establish the proper phase relation for a given degree of coupling to the resonators. For strong coupling and low loss operation the separation between the two resonator frequencies will be substantial, and the circulator will operate over a broadband. With weaker coupling medium loss operation results over a narrow band since the resonant frequency separation is smaller. Thus the bandwidth of the device may be achieved by altering the coupling of the ports to the resonator, |while a change of operating frequency is accomplshed by simply adjusting potentiometer 114. Potentiometer 1S is readjusted only when it is necessary to optimize at a new operating frequency.

Port I is connected by conductive paths 30 and 31 to resonators 10 and 11, respectively. Coupling is achieved in any commonly known manner as, for example, by using symmetrically oriented straps, as illustrated. Path 30 terminates in a conductive strap 40 which is positioned over a portion of the surface of resonator 10 at a fixed distance therefrom, and is grounded at the extremity of the strap. The size and the shape of strap 40 and the distance from resonator 10 may be determined by experimentation so that the optimum coupling to the ferrite sphere and the minimum coupling among the straps are achieved. An identical coupling of Port I to resonator 11 is shown through path 31 and strap 41.

Port II is similarly parallel coupled to resonators 10 and 11 by paths 32 and 33 terminating in grounded straps 42 and `43, respectively. Port III is parallel coupled to resonators 10 and 11 by paths 34 and 35 and grounded straps 44 and 45 in a like manner.

Three straps 40, 42 and 44 are shown coupled to resonator 10 spaced at 120 intervals, but it should be understood that more than three symmetrically positioned straps could be employed with an appropriate resonator. Straps `4|), 42 and I44 are positioned to intersect at a point on resonator 10s spherical axis which is parallel to magnetic field Hdc A and they are electrically isolated from one another at this point by insulation not shown. The arrangement of straps 41, 43 and 45 relative to resonator 11 is identical.

In order to operate as a circulator the rotating modes excited in resonators 10 and 11 must be oppositely directed relative to the ports. In a ferrite sphere utilizing the uniform precession mode, the direction of rotation in space is a function of the direction of the magnetic field Hdc. Therefore, oppositely directed rotation can be established by oppositely directing the magnetic fields Hdc A and H0164; where straps 40, 42 and 44 are ordered similarly to straps 41, 43 and 45, as shown.

Alternatively, the relative direction of rotation of the precession mode can be achieved with an identical spacial orientation of magnetic fields by ordering the straps differently for the two resonators. For example, where the magnetic fields for both resonators have the same sense oppositely rotating modes will be produced if straps 40, 42 and 44 are spaced counter-clockwise in that order around the axis of the magnetic field for resonator 10 while straps 41, 43 and 45 are spaced clockwise in that order around the axis of the magnetic field for resonator 11. In other resonators or where the uniform precession mode is not used the oppositely rotating relationship could be similarly established by conventional techniques.

As in conventional three port circulators, when the phase relation of the two resonant modes is 60 and an input within the bandwidth of the device is introduced, circulator action results. The composite output superposed at the output port contains essentially all of the energy produced at the input port, while the interaction of the modes results essentially in a null at the last port.

The result of adjusting the magnetic fields Hdc A and Hdc B simultaneously by common potentiometer 14 is illustrated in FIG. 2. Decreased magnetic fields cause the resonant frequency jA for resonator 10 to shift down to fA and resonant frequency fB for resonator 11 to shift to fB keeping the phase relation of the two resonant modes almost constant at 60 as illustrated by phase-frequency characteristics 52 and 52 for resonator 10 and characteristics 54 and 54' for resonator 11. The center frequency fo of the circulators transmission characteristic 56 which is inherently midway between the two resonant frequencies fA and fB, would also shift downward in frequency. The shifted transmission characteristic 56 would have a center frequency fo which may be an octave below the initial center frequency fo.

While a rigorous analysis of the invention can be given using the model developed in the above-mentioned article by F ay and Comstock, such an analysis does not give a particularly intuitive understanding of the invention. The following explanation, based upon rst order effects, and neglecting the effects of scattering and higher order modes, is offered as giving a physical understanding of the observable overall performance.

The coupling produced by bodies 10 and 11 can be explained by recognizing that the gyromagnetic material of which they are composed contains unpaired electron spins which tend to line up with the applied biasing field. These spins have an associated magnetic moment which can be made to precess about the D.C. biasing field, keeping an essentially constant moment component in the direction of the biasing field and at the same time providing a moment component which may rotate in a plane normal to the field direction. When a reciprocating high frequency magnetic field of electromagnetic wave energy is impressed upon the moment, the magnetic moment will commence to precess in one rotational sense.

If the shape of each gyromagnetic body is such that the biasing field is substantially uniform throughout the body, a condition referred to in the art as uniform precession exists in which all electrons precess together. The frequency of precession is proportional to the strength of the biasing field. The combined effect of many such electrons and their associated moments produces in the material not only the impressed reciprocating magnetic field but also corresponding reciprocating fields at angles in space to the impressed field, displaced in time from the impressed field by a phase determined by the angle in space, the direction of precession and by the relationship between the frequency of the impressed field and the frequency of precession.

In accordance with the invention, the above-defined impressed feld is excited by driving one of the straps 40 through 45 of FIG. 1 on each body with a current, and

it is therefore excited in a plane normal to the biasing lield in a direction dependent upon the physical placement of the driven strap relative to its biasing field. By converse action the displaced magnetic fields induce currents in each of the undriven straps.

Consider now the phase angle between the driving current and the field induced by it on one hand, and a displaced field and the current which it induces on the other hand. When the frequency of the driving field is equal to the frequency of the gyromagnetic precession (the condition known in the art as gyromagnetic resonance), the displaced field is at a maximum and at a given physical angle away from the driving field has a phase that is delayed'- in time by a phase angle numerically equal to the physical angle. When the frequency of the driving eld is higher than the precession frequency, the displaced field reaches a given phase in its own cycle before it has reached the physical angle corresponding to this phase in the precession cycle. Thus, the phase of the displaced field at a given physical angle is greater than the physical angle itself by the amount that the precession frequency lags the driving frequency. Similarly, when the driving frequency is lower than the precession frequency, the phase of the displaced field is retarded or less than its physical angle. In other words, gyromagnetic resonance exhibits an amplitude and phase characteristic similar to any other resonantcircuit. For this reason it is appropriate to refer to bodies and 11 as resonators and to illustrate their characteristics as in FIG. 2 by phase and amplitude characteristics analogous to these characteristics in other resonant circuits.

In accordance with the present invention, one of the bodies, such as 10, is biased so that its gyromagnetic precession frequency falls below the operating frequency by that amount for which the displaced field at the operating frequency is advanced by a 30 degree phase, and the other of the bodies, such as 11, is biased so that its gyromagnetic precession frequency falls above the operating frequency by the same amount. Thus, in FIG. 2 phase and amplitude characteristics 51 and 52 represent the amplitude and phase respectively of precessional resonance of resonator 10 centered on the frequency fA and characteristics 53 and 54 represent the amplitude and phase respectively of precessional resonance of resonator 11 centered upon the frequehcy fB. The spacing of frequencies fA and fB from the operating frequency fo is such that the impressed field at fo lags in one body and leads in the other by 30 degrees as illustrated by the values of characteristics 52 and 54 at fo. This phase difference is optimized by potentiometer 15, as is discussed above.

The effect that these phases have upon the structure in FIG. 1 may be seen schematically in FIG. 3. Independent ferrimagneti: bodies 10 and 11 are each coupled to three branches 1, 2 and 3 and 1', 2' and 3', respectively. The branches are symmetrically disposed in 120 degree relation to ons. another and branches 1 and 1', 2 and 2' and 3 and 3' are joined in parallel pairs to form ports I, II and III, respectively. Body 10 is biased by D C. magnetic field Hdc A in a direction shown as being into the paper producing a precessional direction from branch 1 to 2 to 3, while ybody 11 is biased by Hdc B in the opposite direction, illustrated as out of the paper, thus producing the opposite precession direction from branch 1' to 3' to 2'.

Assume that the input signal is applied to port I at the operating frequency fo, that Hdc A is lower than Hdc B and that the resulting precession frequency fA in body 10` is less than the frequency fo and the precession frequency fB in body 11 is greater than the frequency ff), parallel connected branches 1 and 1' on each body will excite equal impressed fields in each body which are in phase. Precession in'body 10 will transfer the signal to branch 2 at a phase of 120 degrees due to the angle of branch 2 with respect to branch 1 and with an additional 3'0 degrees as described abve. Similarly, the signal will be induced on branch 3 at a phase of 240 degrees plus an 6 additional 30 degrees. In body 11, however, the signal will appear on branch 3' at a phase of 120 degrees less 30 degrees and at branch 2' at a phase of 240 degrees less 30 degrees. Note, therefore, that the induced signals on branches 3 and 3' are 180 degrees out of phase, and if equal in amplitude, will sum to produce no output at port III. The remaining branches 2 and 2', which form port II, have phases which sum to produce an o-utput representing the signal applied to port I. Thus, there has been a circulation of power from port I to port II with none appearing at port III. If port II had been initially excited the transfer would be to port III with none at port I. It should be noted that the concepts of lag and lead in the above explanation are to some extent arbitrary depending on what is defined as the reference phase. Irrespective of this definition, the direction of circulation will be in the direction of precession in the body of the lowest precession frequency.

The bandwidth of frequencies undergoing the circulator action described is represented by transmission characteristic 56 in FIG. 2. Since the resonance characteristics 51-52 and 53-54 have a sharpness that depends on the degree of coupling between the straps and the bodies (effectively varying the loaded Q of the equivalent resonant circuit), the bandwidth of the circulator can be controlled by varying this coupling at the expense of transmission through the circulator. Thus, decreasing the coupling increases the Q and the slope of phase characteristics 52 and 54. In order to maintain the required 60 phase separation, the spacing between frequencies fA and fB is decreased by adjusting potentiometer 15, which in turn increases transmission and narrows the bandwidth of characteristic 56. Conversely, increasing the coupling fiattens characteristics 52 and 54, increases the spacing between fA and fB and increases the bandwidth.

In all cases it is to be understood that the above described arrangement is merely illustrative of one of the rnany possible applications of the principles of the invention. Numerous and varied other arrangements in`l accordance with the principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

I claim: p

1. A circulator having at least three ports comprising:

a first gyromagnetic resonator coupling said ports and capable of supporting an electromagnetic field rotating in a first sense relative to said ports,

a second gyromagnetic resonator coupling said ports substantially in parallel with said first resonator and capable of supporting an electromagnetic field rotating in a sense opposite to said first sense relative to said ports,

a first of said ports exciting each of said resonators with electromagnetic wave energy in a first phase in the respectively rotating field of each, each of said other ports coupling with both said resonators with said rotating fields in phases in each resonator displaced by diferent amounts respectively from said first phase when measured in the sense relative to said rotation in that resonator so that energy coupled from said first resonator combines with energy coupled from said second resonator to produce a sum of said coupled energy in one of said other ports and to produce a mutual cancellation of said coupled energy in the other of said other ports.

2. A circulator as claimed in claim 1 including means for producing D.C. magnetic fields of different strengths polarized along an axis of each gyromagnetic resonator.

3. A circulator as claimed in claim 2 wherein said means for producing D.C. magnetic fields bias one of said gyromagnetic resonators to a precession frequency above the operating frequency of said circulator and bias the other of said bodies to a precession frequency below said operating frequency.

4. A circulator having at least three ports comprising:

first and second gyromagnetic resonators each capable of supporting an electromagnetic field rotating in a given sense,

first means for exciting each of said resonators with electromagnetic wave energy in a given phase,

means for each resonator for separately coupling with said rotating eld in second and third successively different phases each displaced respectively from said given phase in the same sense relative to said rotation,

means for combining energy coupled from said first resonator in said second phase with energy coupled from said second resonator in said third phase to produce a sum of said coupled energy,

and means for combining energy coupled from said second resonator in said second phase with energy coupled from said rst resonator in said third phase to produce a mutual cancellation of said coupled energy.

5. The circulator of claim 4 wherein said means [for separately coupling comprises means separately displaced References Cited UNITED STATES PATENTS 2,818,501 12/1957 StaVis 333-l.1 3,274,519 9/ 1966 Nathanson. 3,435,385 3/1969 Cohen.

HERMAN KARL SAALBACH, Primary Examiner P. L. GENSLER, Assistant Examiner U.S. C1. X.R. 333-73 

